Nowadays, a majority of apps require you to sign up with your email or even worse your phone number. If you have a phone number attached to your name, meaning you went to a cell service/phone provider, and you gave them your ID, then no matter what app you use, no matter how private it says it is, it is not private. There is NO exception to this. Your identity is instantly tied to that account.
Signal is not private. I recommend Simplex or another peer to peer onion messaging app. They don’t require email or phone number. So as long as you protect your IP you are anonymous
Did you look it up?
Yes, as I said, the government can tell if you use Signal or not by asking Signal (by providing Signal a phone number and asking if they have a record of it).
It’s not anonymous in that sense, but it is still private because your messages cannot be revealed by such data requests.